The current proposal for uplink access in the 3rd Generation Partnership Project (3GPP) Long-Term Evolution (LTE) system uses Interleaved Frequency Division Multiplexing (or Frequency Division Multiple Access (FDMA)) on the uplink as the multiple access scheme. This scheme uses subcarriers that are evenly spaced on the frequency grid or clustered together; this choice can lower the crest factor (i.e., Peak to Average Power Ratio) of the transmitted signal compared to Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA). With even spacing of subcarriers across the frequency grid, a signal with a significant amount of frequency diversity is obtained, which is useful for cases when no information on the radio channel is available. The clustered subcarriers are advantageous in the case when the channel is known to be good over the portion of the band where these subcarriers are used. The physical layer interface for the 3GPP LTE system is specified in the document “Physical layer aspects for evolved Universal Terrestrial Radio Access (UTRA)”, 3GPP TR 25.814 V7.1.0 (2006-09) Technical Report, which is published by the 3GPP, Technical Specification Group Radio Access Network.
For example, FIG. 1 shows an exemplary uplink transmission scheme 100 that always uses pilots 102 before the data 104 in accordance with the signal design as described in the 3GPP contribution titled “Uplink Transmission and Multiplexing for EUTRA”, presented at the 3GPP TSG RAN WG1 Ad Hoc on LTE, Sophia Antipolis, France. (2005-06-20) document number R1-050605. Part of the uplink transmission uses sixteen evenly spaced subcarriers over two timeslots—the first slot 106 contains pilot symbols 102 and the second slot 108 contains data symbols 104. At least one pilot symbol 102 is needed per subcarrier since the subcarriers are distributed and the channel can change significantly from one used subcarrier to the next.
One problem with the existing solution as described in the previous paragraph is that the pilot symbols are extremely vulnerable to interference. It is conceivable that a co-channel user in a different cell is using the same set of subcarriers to send uplink information. The pilots of the desired user then encounter a significant amount of co-channel interference and this can corrupt the channel estimates that are obtained by the receiver from the received pilot symbols. In turn, this can lead to erroneous decoding of the data even if it is protected with a strong error correction code. Thus, there is a need to improve the signal design for the uplink so that it is more resistant to co-channel interference.
In another example, FIG. 2 shows an uplink transmission scheme 200 where the pilot symbols 202a and 202b are divided into two parts over the slot, but are wider in frequency by a factor of two (and thereby narrower in time by a factor of two) compared to the data symbols 204a, 204b, 204c and 204d. Two mobile stations (users) share the pilot and data symbols. The data slots of the two mobile stations are transmitted over two adjacent sets of interleaved subcarriers (e.g., 204b and 204c). For example, user 1 may use the pilot and data symbols identified by the numeral 1, and user 2 may use the pilot and data symbols identified by numeral 2. In this case too, it is seen that either user is susceptible to co-channel interference on the pilots from other users that may be using the same set of subcarriers in a different cell. Thus, there is a need to improve the signal design in the case also.